Phone started getting hot spot on device

The patch appears to be where the SIM/memory card is, and it gets very hot to the touch. It has happened when the phone is idle and does not appear to happen when I am doing something to access that area (e.g. playing movies or making calls). It's hot enough to go through to the glass/screen side. Turning the device off and leaving it for 10 minutes lets it cool, and it doesn't happen again for a couple of days. The phone is around 1 year old, and has only started doing this in the last couple of weeks.

When that happens, check your battery stats to see if any app seems to be using an excessive amount of battery.

A corrupt SD card can also cause behavior like that, because the system's Media Scanner can get hung up on a corrupt file, thus causing the CPU to work overtime. Unmount the card in Settings>Storage​, remove it, and reboot. See if the overheating recurs.